I posted a circuit that does all the things I mentioned as important. That's what we do here, discuss electronics. Except for some people, who would rather whine about personalities.
Quarrel? No, it would probably do for his application. More critical apps, probably not.
John
I hope that's a good thing? ...Jim Thompson
-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
Remember: Once you go over the hill, you pick up speed
Right. On-topic rules!
-- Joe
Very nice, especially the clever use of the 1M for hysteresis as well as part of the input voltage divider. :-) _____ The only "problem" I see it that RESET goes high when the +28V line gets to 24.5V instead of waiting until it's 28V.
=A0 =A0 ...Jim Thompson
Being sloppy has its merits, at least so we slobs think, but don't try to fix us - we're born that way.
Oh I see - you meant "is being J.T. a good thing?". Evidently it's good for J.T. I rather suspect it's bad for the eco-system - a carbon footprint as big as Saskatchewan ;-)
-- Joe
No, that makes sense. You wouldn't want it to hang forever because the supply is 27.9, or the resistors aren't perfect.
John
I assure you that I am not error-free... I just beat my designs to death, find the subtle fault mechanisms, and fix them before they reach manufacture. ...Jim Thompson
Drama aside, there are several good ideas here (along with a few useless responses). What I want to do is use a POR IC, but I'm having difficulty figuring out a good way to get it to monitor 28V as well as
5V (and not time out in less than three seconds). I've been looking for a device that's made to do that, but I can't find one. Maybe this circuit with the MAX809 and transistor is the best way to do that? Thank you everyone for your ideas.
What is the engineering basis for 3 seconds delay? What do you mean by "monitor"??? Relative deviation from nominal 5% , 10 %, 15% or what is it, and what is the engineeriing basis for your demands? There is a device that will do it all and it's called a replacement power supply.
John Larkin gave you a circuit to monitor both supplies. If you want a
3-second delay from that, a single-shot (555) is the easiest way to go.
I don't think he needs a long delay. Once both supplies are up, he's probably good to go.
But 555? NOOOOOooooooooooo.......
John
But he wanted a delay.
But he wanted a *long* delay. You wanna use a PIC? ;-)
(you're starting to sound like Slowman with the triskapentaphobia thing ;-)
I think that's because the +5 supply came up fast but the 28 volt supply came up slow. I think. My circuit has a short delay after both have come up.
John
Well, if 100-200msec is enough, there are lots of single chip solutions- 2 resistors (for the 28V) plus the chip (preset for use on a 5V supply). Except that he wants it to be through-hole, which is much more limiting. So, he should change that 'requirement' too.
eg.
SOT-23-5. A bit bulky, but he can use 0402 or 0201 resistors to keep the footprint within reason.
Best regards, Spehro Pefhany
It appears to me that you are looking more for a project to do than simply going for the easiest method and getting it done.
It's a very simple thing, if you just want to get it started with both psu operating then connecting the outputs to printer or what ever it was you were powering..
A 24V DC DP Relay being driven from the 28 volt supply with a R in series.
You feed the 28 volts through one set of contacts (NO) and the 5 Volts which is on before the 28V as you indicated, also through the other set of (NC) contacts. The other side of the contacts simply connects to the Uc board at the same time with both supplies up and running. So you get both 5 and 28 volts simultaneously to the uc board. The current reset circuit should work.
Now if that isn't simplicity, I don't know what is. Anything else is just a waste of time for the piece of equipment you are nursing, unless of course, as I said before, you're making a project out of it.
If you really do want to go the way of employing your own RESET circuit, then may I suggest, to keep it simple, use a 555 timer. The timer can operate from the 5 volt supply and use the Trigger or Threshold, depending which way you need to reset to work, from a R divider of the
28 volt supply, this will give you a very simple voltage comparator switch with an output that can swing 200 ma's if needed.At work, time is money. The simplest solutions gets the prize every time.
Jamie.
That creates the wrong impression. JF posts more circuits - by a wide margin - than anyone else here. His error percentage is very low, likely lower than most other posters. The few times he makes an error, he (almost always) corrects it himself before anyone else spots it, as you alluded to. I don't see anyone else publicly admitting to, and correcting, errors they make.
Ed
You're welcome!
Being able to roll your own allows some nice gimmicks in power monitoring (and sequencing if needed by your system). ...Jim Thompson
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